[PATCH] gc.c (Init_GC): clamp M_ARENA_MAX=2 for glibc malloc

[PATCH] gc.c (Init_GC): clamp M_ARENA_MAX=2 for glibc malloc

[Eric Wong]

glibc malloc creates too many arenas and leads to fragmentation.
Given the existence of the GVL, clamping to two arenas seems
to be a reasonable trade-off for performance and memory usage.

Some users (including myself for several years, now) prefer only
one arena, now, so continue to respect users' wishes when
MALLOC_ARENA_MAX is set.

Thanks to Mike Perham for the reminder [ruby-core:86843]
---
 configure.ac |  1 +
 gc.c         | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/configure.ac b/configure.ac
index c1f6525634..c9c24c6bc6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1788,6 +1788,7 @@ AC_CHECK_FUNCS(lstat)
 AC_CHECK_FUNCS(lutimes)
 AC_CHECK_FUNCS(malloc_usable_size)
 AC_CHECK_FUNCS(malloc_size)
+AC_CHECK_FUNCS(mallopt)
 AC_CHECK_FUNCS(mblen)
 AC_CHECK_FUNCS(memalign)
 AC_CHECK_FUNCS(memset_s)
diff --git a/gc.c b/gc.c
index bd52966bf1..613c184083 100644
--- a/gc.c
+++ b/gc.c
@@ -9755,4 +9755,14 @@ Init_GC(void)
 #undef OPT
 	OBJ_FREEZE(opts);
     }
+
+#if defined(HAVE_MALLOPT) && defined(M_ARENA_MAX)
+    /*
+     * Ruby doesn't benefit from many glibc malloc arenas due to GVL,
+     * remove or increase when we get Guilds
+     */
+    if (!getenv("MALLOC_ARENA_MAX")) {
+        mallopt(M_ARENA_MAX, 2);
+    }
+#endif
 }
-- 
EW